1. Field of the Invention
The invention relates to a web that is a continuous material in the form of a flexible sheet, a web conveying apparatus, and a web conveying control method.
2. Description of the Related Art
A web that is a continuous material in the form of a flexible sheet, such as a plastic film, a metal film, and a continuous paper sheet is conveyed while being supported on a plurality of rollers. Cost reduction is required for a conveying apparatus that conveys such a web in order to increase productivity, and the increase in conveying speed, efficiency and accuracy are necessary.
A problem arising when a web is conveyed is that a transverse position of the web fluctuates and a wrinkle is formed due to misalignment of a guide roller, which is a roller for controlling the transverse position of the web, and free rollers provided upstream or, downstream of the guide roller, or because of disturbance fluctuations such as web tension or speed fluctuations. Accordingly, in a web conveying apparatus disclosed in Japanese Patent Application Publication No. 06-239506 (JP-A-06-239506), the formation of wrinkles is inhibited by detecting a web edge with an edge sensor or the like and controlling the transverse position of a web with good accuracy by using a guide roller.
However, with such a control of transverse position, because the guide roller is intentionally moved, it can cause misalignment of rollers. As a result, slackening or undulations occur in the width direction of the web, and in a case where such undulations are large, the formation of wrinkles cannot be sufficiently inhibited only by controlling the fluctuations of transverse positions
For example, a conveying apparatus 90 of related art that is shown in FIG. 11 includes free rollers 95a and 95b that support and convey a web 20, a guide roller 93 provided between the free rollers 95a and 95b, an edge sensor 92 that detects a transverse position of the web 20, a control unit 91 that calculates a correction amount of the transverse position of the web 20, and a transverse position correction unit 94 that controls the guide roller 93 on the basis of the calculated transverse position correction amount. As shown in the figure, because the guide roller 93 is intentionally moved, the transverse position of the web 20 is controlled more than necessary. As a result, undulations can occur in the width direction of the web, and when the undulations are large, a wrinkle may be formed.
Furthermore, as the web conveying speed rises, the above-described transverse position control is actuated in a transient mode. In this case, the formation of wrinkles also cannot be sufficiently inhibited. Thus, in a case where a value detected by an edge sensor deviates greatly from the target value thereof, fluctuations of web position increase abruptly where such deviation is to be corrected, and the transverse position correction is actuated in a transient mode. Furthermore, even when the web position fluctuates gradually rather than abruptly, the transverse position correction is actuated in a transient mode because the web position itself fluctuates greatly. In a case where the control of transverse position is thus performed in a transient mode, the web can move, following the transverse position correction control, in the direction such that parallelism of the rolls can be greatly degraded and a wrinkle may be formed in the web.
Japanese Patent Application Publication No. 2007-326657 (JP-A-2007-326657) discloses a web conveying apparatus in which undulations, which indicate that a wrinkle may be formed, are detected with image analysis means on the basis of events such as the occurrence of undulations or slackening in the web width direction, prior to the formation of wrinkles in the web, and a guide roller is driven in the direction of canceling such undulations.
However, with the technology disclosed in JP-A-2007-326657, an expensive detection device such as image analysis means is used for detecting the undulations, and using such an image analysis means increases the equipment cost. Accordingly, a demand has been created for a technology that can effectively prevent the defects (wrinkles caused by the occurrence of waving) that can occur, while minimizing the increase in equipment cost.
Furthermore, a technology is needed that can detect a defect at a low cost when such a defect has occurred. Presently, as the number of inspection processes increases, the outflow of defects occurring in these processes unavoidably affects the entire equipment cost. Therefore, in order to prevent the outflow of defects from the processes themselves, it is necessary to prevent the formation of wrinkles in each process and also to, detect the wrinkles that have been formed and eliminate the defects in the processes themselves.
Furthermore, the web edge is not necessarily in the form of a straight line and can have high-frequency undulations due, for example, to burring occurring during slitting or because of curling occurring during rolling. A problem arising where web position correction control is executed with respect to a web having such undulations is that the undulations cause transient actuation in the transverse position control and the formation of wrinkles cannot be sufficiently inhibited.